| Hi David Although you say they grow well there, there are sometimes circumstances that aren't favorable for some specific plants(ie ours). That red-brownish-coloured leaves remind me either of sun damage or mite infection. The damage has probably occured in the summer and the leaves then turned slightly whitish.Isn't it?
Here in Greece Buxus is a native shrub-small tree. But I have noticed them growing only in mountainous areas, in a high attitude.They prefer cool summers. Your Buxus probably don't like that situation.The pots are exposed to the sun(southern exposure), making the roots burn at noon.Then the whole plant suffers. An other reason for their reddening is a mite infection.I have seen many Buxus turning red after a long period of being infected with mites, either Tetanychus cinnabarinus or Tetranichus urticae.The symtoms start by tiny yellow bites on the leaves.With a magnifying glass you can see the mites underneath the leaves.The leaves later turn yellow and in the long run they sometimes turn red and finally fall off the plant. Now the treatment.
Exclude the possibility of mite infection
you can be helped by looking the following sites http://mint.ippc.orst.edu/suckinginsects.htm
http://www.plantlilies.com/read/mites.html If they have indeed mites it may be difficult to see adults now.Only eggs can be seen(by experts though) usually one to two sprays with a pesticide can get rid your plants of those mites(if there is infection) Now, if there is no infection, then your plants simply suffer being in that location, in the southern exposure it would be difficult for them to thrive. Move them to a shady site(northern exposure) or under a tree so as to be cool in the summer. If you want you can plant them in the ground.The soil in the ground has smore favorable temperatures for the roots to develop.It is not overheated like that in pots. Generally Buxus are plants that grow slow and have also a slow rate of correspondance to everything, such as infection, fertilization etc.So be patient and they will return to normal. It will help also to fertilize them with Iron(chelate).Since they are under so much stress it will relieve some of it. Good luck | 
